4 Mapping background NO2 and NOx concentrations
4.1 Current annual mean concentrations

Maps of estimated annual mean background NO2 and NOx concentrations for 1996 are presented in Figure 4.1 and Figure 4.2. These maps are from our earlier report on background concentration maps (Stedman, 1998), where more details of the mapping methods can be found. Our general approach to mapping background concentrations has been described in Stedman (1998) and Stedman et al (1997).

Measured annual mean background concentrations have been considered to be made up of two parts:

• A contribution from relatively distant major point and area sources such as power stations or large conurbations. Measurements from monitoring sites well away from local sources, from sites within the Department of the Environment Transport and the Regions' (DETR) rural networks, for example, provide good indications of the spatial variation of concentrations due to distant sources.
  
• A contribution from more local emissions. We have found that estimates of low level emissions in an area of 25 km2 centred on an urban background monitoring site location can be used to derive this local contribution.
  

The difference, diff, between measured ambient NOx and NO2 concentrations at urban automatic monitoring sites (not roadside or industrial sites) and an underlying rural concentration field is calculated where monitoring data are available.

diff = measured annual mean urban concentration - mapped rural concentration

A regression analysis is then performed to find coefficients, km, for the relationship between diff and estimated NOx emissions in the vicinity of the monitoring sites taken from the National Atmospheric Emissions Inventory (NAEI) (Salway et al, 1997, Goodwin et al, 1997):

diff = km.emissions

This coefficient, which is the equivalent of an empirical box model coefficient, can then be used to derive a map of annual mean concentrations from a combination of a rural map and emissions inventory estimates. Thus automatic monitoring data is used to calibrate the relationship between ambient air quality and emissions inventories.

estimated concentration (ppb) = rural map (ppb) + km. emissions (Tonnes NOx as NO2 per 25 km2 per year)

A map of rural NO2 concentrations has been interpolated from monthly measurements of NO2 by diffusion tubes at DETR Acid Deposition Secondary Network sites (Vincent et al, 1998). A map of rural NOx concentration has been derived from this NO2 map by multiplying by 1.2 (the measured ratio at the Lullington Heath monitoring site, which is very similar to that found at other rural automatic monitoring sites). The empirically derived coefficients used to calculate the maps of background NOx and NO2 concentrations for 1996 are listed in Table 4.1.

Table 4.1 Coefficients used to calculate 1996 background annual mean maps

	km
NO2	5.920
NOx	17.349

The highest estimated background concentrations are in the centres of the large cities. The maximum concentrations are 43 ppb for NO2 and 97 ppb for NOx. Areas strongly influenced by emissions from busy motorways are also evident.


4.2 Predicting annual mean concentrations for 2005 and 2009

Maps of estimated background concentrations for 2005 and 2009 are also shown in Figure 4.1 and Figure 4.2. Annual mean background NO2 concentrations in the centres of large cities, other than London, in 2005 are expected to be in the range from 15 to 21 ppb, within the NAQS objective. Concentrations in inner London are expected to be in excess of 21 ppb; 29 ppb is the maximum concentration represented on the map. Concentrations will be lower still in 2009 but the maximum predicted concentration in inner London is 26 ppb, still well above the EU annual mean limit value.

Predicted concentrations in inner London are, however, extremely sensitive to the NOx to NO2 conversion assumptions within the mapping, as discussed in section 3. Preliminary results from a non-linear model suggest that maximum predicted background concentrations of NO2 in 2009 in inner London will be lower than 21 ppb. Concentrations in 2005 are, however, expected to be greater than 21 ppb in inner London. It is likely that the projections of background NO₂ concentrations presented here represent a worst case analysis, as discussed in section 3.2.


4.3 Predicting background NO2 concentrations for individual monitoring sites

The emissions projections that have been used to calculate the maps of estimated background NO2 concentrations for 2005 and 2009 can also be applied to individual site measurements for 1996 or 1997 to provide projections which do not have the uncertainty associated with the mapping methods. Projections based on 1996 and 1997 measured annual mean NO2 concentrations at background sites are listed in Table 4.2 and Table 4.3.

Measured NO2 concentrations at background sites were higher than 21 ppb at most sites in 1996 and 1997. Business as usual values for 2005 are lower than 21 ppb for all sites except those in central London, which is consistent with the mapping analysis for 2005. Projected concentrations for 2005 at London Hillingdon and London Hackney are also greater than 21 ppb but these are not typical background sites. Both of these sites are influenced by nearby major road: London Hillingdon is 30 m from the M4 and London Hackney is 20 m from the A107.